1. Field of the Invention
The present invention relates to a coated metal fluoride particle suited for use in dental materials for the prevention of caries by strengthening the tooth substrate, and to a dental composition incorporating the metal fluoride particle.
2. Discussion of the Background
The effect of fluoride ions to strengthen the tooth substrate in dental treatment is already well known, and the treatment of the tooth substrate with fluoride ions to prevent and/or inhibit tooth caries is performed routinely.
In recent years, restorative resins have been employed as materials for tooth restoration. However, in the unfortunate case that a gap forms between the tooth substrate and the restorative material, leaks can occur around the margins and bacteria can penetrate into the gap between the tooth substrate and the restorative material and cause secondary caries.
Dental restorative resins that contain a fluoride compound have been proposed with the aim of preventing secondary caries by strengthening the tooth substrate through the fluorination of the tooth substrate around the wall of the cavity by fluoride ions which elute from the fluoride compound (Japanese Patent Application Publication No. Sho 48-80151 and No. Sho 50-49358).
Conventionally, metal fluorides such as sodium fluoride, fluorides of ammonium salts such as ammonium fluoride, and fluorine containing glasses such as fluoroaluminosilicate glass are known to release fluoride ions. However, with metal fluorides and fluorides of ammonium salts, although the amount of released fluoride ions is high and subsequent fluorination of the tooth substrate by the release of fluoride ions into the surrounding tooth substrate can be realized, there is the problem that a large decrease in the mechanical properties and adhesive performance of the restorative resin itself accompanies the elution of fluoride ions, causing further fracture and shedding of the restorative resin. On the other hand, with fluorine containing glasses, although there is no decrease in the mechanical properties or adhesion performance of the dental restorative resin incorporating the glass, there is the problem that the amount of fluoride ions eluted is small.
In order to provide a fluoride ion releasing dental composition having excellent durability and high practical value, there is a need to develop a fluoride ion releasing material which elutes fluoride ions in large amounts but which does not reduce the mechanical properties or adhesion performance of the restorative material itself.
The inventors of the present invention considered that when a fluoride compound is incorporated into a restorative resin to supply fluoride ions to strengthen the tooth substrate for preventing secondary caries, it is desirable that the fluoride compound elutes fluoride ions in large amounts. From this point of view, the inventors of the present invention selected in particular metal fluorides from the large number of different fluoride compounds, and studied their incorporation into dental restorative materials.
When the relationship between the amount of fluoride ions eluted and the decrease in mechanical properties and adhesion performance was investigated for dental restorative materials incorporating metal fluorides, it was observed that although the amount of fluoride ions eluted increased with an increasing amount of metal fluoride incorporated, the mechanical properties and adhesion performance is adversely effected with the increasing amount of metal fluoride incorporated. From these findings, it was considered that it would be possible to inhibit the decrease in mechanical properties and adhesion performance of the restorative resin by controlling the speed of elution of the metal fluorides incorporated therein.
However, if the sole objective is to simply control the speed of elution of fluoride ions from the dental restorative resin having the metal fluorides incorporated therein, it can be achieved by the conventional technique of microencapsulation. As described in "Kobunshi Daijiten" (1994, Maruzen Kabushiki Kaisha) and "Shinpan Kobunshi Jiten" (1988, Asakura Shoten), microencapsulation involves coating the surface of a core material such as small solid particles, liquid drops, or gas bubbles to seal it, thereby protecting the core material from the external environment, and controlling the speed at which the core material is eluted to the outside. By adopting a metal fluoride as the core material and coating the surface thereof, it is possible to control the speed of elution of the metal fluoride.
The technique of microencapsulating fluoride compounds is disclosed in Japanese Patent Publication No. Hei 2-31049. However, it is used there with the aim of preventing the elution of fluoride ions from the fluoride compound in order to inhibit reactions with other components in the same system. It is thus used as "protection for the core material" as mentioned in the above description of the microencapsulation technique, which is completely different from the objective in the present invention which requires that the coating acts to actively cause fluoride ions to be eluted.
Furthermore, there is disclosed in Japanese Patent Application Publication No. Sho 58-99409 a fluoride-containing aluminosilicate glass powder whose surface in coated with a soluble polymer. When a powder coated with a soluble polymer is used in a dental restorative material, under the damp conditions found inside the oral cavity, the soluble polymer is washed away by saliva or water taken into the mouth during eating and drinking, which necessarily limits its coating function with respect to the powder over a long period of time and is thus unable to fulfill the objective of the present invention, which is to ensure retention of mechanical properties and adhesion performance of a restorative material containing fluoride particles.
It is common for the fluoride compound to be incorporated into the dental composition in its powder form, rather than being dissolved in the dental composition (Japanese Patent Application Publication No. Sho 48-80151 and No. Sho 50-49358). It is also common for the powder filler to be subjected to a surface treatment such as silane coupling treatment before being incorporated into the dental composition. Of course, the treatment of the metal fluoride with a silane coupling agent would easily be considered, and it can be anticipated that the treatment of the metal fluoride with a silane coupling agent would control elution of the metal fluoride. However, the effect desirable in the present invention cannot be achieved simply by subjecting the metal fluoride to silane coupling treatment.
Studies by the inventors of the present invention with respect to the problem of actively promoting the release of fluoride ions when incorporating a metal fluoride into a dental composition for preventing secondary caries via fluoride ions, whilst avoiding any decrease in the mechanical properties and adhesion performance of the dental restorative resin accompanied with the elution of fluoride ions, have shown that the two requirements cannot be satisfied at the same time with the fluoride compounds obtained by conventional techniques, and that there are thus problems with applying the conventional techniques to dental compositions in the present invention.
The inventors of the present invention carried out research into producing a fluoride particle by which the contradictory requirements in the prior art (i.e., avoiding any inhibition of the speed of elution of the fluoride ions whilst inhibiting the decrease in the mechanical properties and adhesion performance of the dental composition containing the fluoride compound) could both be satisfied. As a result, they found that this objective could be realized by a fluoride particle obtained by coating a metal fluoride with a polysiloxane.
At the start of their research, the inventors of the present invention anticipated that this would have the same result as the conventional microencapsulation technique, i.e., that the elution of the metal fluoride would be inhibited by coating the metal fluoride particle with a polysiloxane, whereby the decrease in the mechanical properties and adhesion performance would also be reduced. However, detailed studies by the inventors of the present invention showed that, contradictory to this expectation, the amount of fluoride ions eluted did not decrease when the metal fluoride was coated with a polysiloxane, and that in fact the amount of fluoride ions was rather increased by coating with a polysiloxane.
This favorable phenomenon that not only was it possible to inhibit any decrease in the mechanical properties and adhesion performance of the dental composition by microencapsulating the metal fluoride with a polysiloxane compound, but also that the amount of fluoride ions eluted could be increased, could not have been anticipated from conventional teachings.
Although the technique of coating inorganic powders with a polysiloxane is disclosed in Japanese Patent Application Publications No. Hei 7-331112 and No. Hei 8-3473, there is no mention whatsoever in these publications of using metal fluorides as the inorganic powder. Furthermore the teaching of these publications is that an improvement in the bonding force between the inorganic powder and the resin and another improvement in the adhesion between inorganic powder particles can be realized with a mixture of an inorganic powder and a resin, from which it would not have been possible to infer the objective of the present invention.